In many applications of single mode optical waveguides, e.g. gyroscopes, sensors and the like, it is important that the propagating optical signal retain the polarization characteristics of the input light in the presence of external depolarizing perturbations. This requires the waveguide to have an azimuthal asymmetry of the refractive index profile.
A slight improvement in the polarization performance of single mode optical waveguides is achieved by distorting the fiber core symmetry as a means of decoupling the differently polarized waves. Two such optical fiber waveguides are disclosed in U.S. Pat. No. 4,184,859 and in the publication by V. Ramaswamy et al., "Influence of Noncircular Core on the Polarisation Performance of Single Mode Fibers," Electronis Letters, Vol. 14, No. 5, pp. 143-144, 1978. However, the Ramaswamy publication reports that measurements on borosilicate fibers with noncircular cores indicate that the noncircular geometry and the associated stress-induced birefringence alone are not sufficient to maintain polarization in single mode fibers.
The invention disclosed in U.K. patent application G.B. Pat. No. 2,012,983 A is based upon the recognition that orthogonally polarized waves are more efficiently decoupled in a waveguide that is fabricated in such a manner as to deliberately enhance stress-induced, or strain birefringence. That patent teaches that such behavior is accomplished by introducing a geometrical and material asymmetry in the preform from which the optical fiber is drawn. The strain-induced birefringence is introduced by at least partially surrounding the single mode waveguide by an outer jacket having a different thermal coefficient of expansion (TCE) than that of the waveguide and a thickness along one direction that is different from its thickness along a direction orthogonal to the one direction. For example, the preform may be a three-layered structure comprising an inner core region surrounded by a cladding layer which is in turn surrounded by an outer jacket layer having a TCE different than that of the cladding layer. Diametrically opposed portions of the outer layer are ground away, and the resultant preform is drawn into a fiber approximating a slab configuration in which the thicknesses of the outer jacket layer are different in two orthogonal directions. A similar result can be accomplished by constructing the preform from an inner core region, a cladding region and two outer jacket layers oppositely disposed along the longitudinal surface of the preform. Difficulty can be encountered in the manufacture of that type of preform since stress is built up in the outer layer. When grinding the outer layer or when cutting slots therein, the built-up stress has a tendency to cause the preform to break. Assuming that a fiber can be drawn from the preform, the stress-forming outer layer is far removed from the fiber core, and therefore, the effect of the stress on the core is minimal.
In one embodiment of G.B. Pat. No. 2,012,983 A represented by FIGS. 10-15, a relatively thick substrate tube forms the outer portion of the optical fiber. In order to impart to the fiber the desired characteristics, either the inner or outer surface of the substrate tube is non-circular. Because at least a portion of the substrate wall must be relatively thick, the efficiency of deposition is adversely affected. Also, since the substrate tube forms the outer, compressive layer of the riber, commercially available tubes may not be usable in the process unless they fortuitously possess the desired expansion and/or viscosity characteristics of the resultant fiber outer layer.